Fluorine-containing copolymers are excellent in such properties as heat resistance, chemical resistance, weather resistance, and stain resistance and used in various fields including semiconductor fields, automobile fields, architecture fields, electric and electronic fields, chemical plant fields, and medical fields.
Various ways have been studied for further improving properties of fluorine-containing copolymers such as heat resistance, mechanical characteristics, and radiation resistance.
One known way for modifying fluorine-containing copolymers is to irradiate them with radiation. Generally known examples of methods for such modification include methods in which fluorine-containing copolymers are heated to a temperature not lower than the melting point and irradiated with radiation (Patent Literatures 1 and 2).
However, when a molded article obtained by molding a fluorine-containing copolymer is heated to a temperature higher than the melting point and irradiated with radiation, the molded article disadvantageously lose its shape. The irradiation with radiation also promotes degradation of fluororesin, failing to sufficiently provide the desired mechanical characteristics.
Patent Literature 3 discloses a method of producing a modified fluororesin. The method includes crosslinking the resin, without preheating, by irradiating ionizing radiation in the designated exposure dose irradiating at a high dose rate of not lower than 100 kGy/sec with a dose of 200 kGy to 100 MGy of ionizing radiation produced from a particle accelerator, whereby achieves improvement in heat resistance and chemical resistance in a simple manner and in a short period of time.
Patent Literature 4 teaches that thermal degradation resistance and compression strain resistance of a fluororesin are improved by heating the fluororesin to a temperature within the range from 0 to 150° C. or from 0° C. to the crystal dispersion temperature, irradiating the heated fluororesin with a dose of 5 Gy to 500 kGy of ionizing radiation, and holding the irradiated fluororesin at a predetermined temperature for a predetermined time.
Patent Literatures 5 to 7 disclose irradiation of a tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymer or a composition containing the copolymer with a dose of 10 kGy or more of ionizing radiation at an irradiation condition of not higher than 100° C.
Patent Literature 8 discloses another method of producing a modified fluororesin molded article. The method includes applying a fluororesin to a metal substrate that does not bond well to the fluororesin, crosslinking the fluororesin by irradiating it with ionizing radiation at 200° C. to 400° C., and peeling or separating the fluororesin from the substrate.
Patent Literature 9 discloses a method of producing a composite material which includes a crosslinked fluororesin layer excellent in abrasion resistance and adhesion to the substrate. The method includes: forming a fluororesin layer on a substrate, firing the fluororesin layer by heating up to 150° C. higher than the melting point; adjusting the temperature of the fired uncrosslinked fluororesin layer to a temperature within the range from a temperature 60° C. lower than the melting point (Tm) of the fluororesin to a temperature 1° C. lower than the Tm; and crosslinking the fluororesin by irradiating it with radiation.
Patent Literature 10 discloses a material coated with a modified fluororesin. The material has a crosslinked fluororesin film on a substrate which is thermally stable at a temperature not lower than the melting point of the fluororesin to be crosslinked. Here, the crosslinked fluororesin film is obtained by crosslinking a fluororesin at a temperature within the range from 250° C. to 400° C. with ionizing radiation.